Hitherto distance measurement is one of basic physical quantities measurements, and many methods have heretofore been employed therefor. “Distance” is equal to “length” in a physical dimension and they are often overlapped in concept. When an object to be measured is measured by a measuring instrument without bringing the measuring instrument into contact thereto, the measurement is often considered to be a “distance”, while, when the object is measured by a measuring instrument which is brought into contact thereto, the measurement is often considered to be a “length”. A basic instrument for measurement of length is a rule, which is graduated according to a predetermined reference.
Japanese Unexamined patent Publication JP-A-3-144306 (1991) discloses a prior art related to a length-measuring instrument in which a slit is formed in a waveguide so as to extend in the axial direction, electromagnetic standing waves are generated in the waveguide, and a slider which inserts a plurality of probes from the outside into the inside of the waveguide through the slit is moved in the axial direction to find the position of the slider based on the amplitude of the standing waves detected by the probes.
The present applicant further discloses in Japanese Unexamined Patent Publication JP-A-11-230734 (1999) a prior art related to a linear encoder in which standing waves are formed in a linear conductor passage with the use of frequency-modulated waves whose frequency periodically changes, and the layer position of the conductor passage is measured based on a correlation between the modulated signals and the envelope of standing waves in the conductor passage.
The present inventors have further discovered that electromagnetic standing waves can be formed in a propagation medium such as space that exists relative to an object to be measured, have developed the idea of using the standing waves applied to the conductor passage in Japanese Unexamined Patent Publication JP-A-11-230734 (1999), and have proposed in Japanese Patent Application No. 2001-237280 a technology capable of measuring the distance in a non-contacting manner.
In the prior arts disclosed in Japanese Unexamined Patent Publication JP-A-3-144306 (1991) and JP-A-11-230734 (1999), the waveguide or the conductor passage corresponding to the rule is installed between a position that serves as a reference of measurement and the object to be measured, and must be brought into mechanical contact with the object to be measured. The technology proposed in Japanese Patent Application No. 2001-237280 uses standing waves which are formed in electromagnetic waves or the like whose propagation medium is, for example, space, and accordingly it is made possible to measure a distance to an object in a non-contacting manner. Therefore, this technology can be utilized as a radar which is mounted on a mobile body such as an automobile.
According to the technology proposed in Japanese Patent Application No. 2001-237280, the frequency of electromagnetic waves or the like for generating the standing waves is varied to find a detection signal function that represents a relationship between the amplitude of the detected standing waves and the frequency, and a frequency at which the detection signal function assumes an extreme value is related to a distance to the object to be measured. As will be described later, however, changes in the detection signal function decrease near the extreme value, and it becomes difficult to correctly specify the position of the extreme value with the result that enhancement of resolution has its limitation. In the practical measurement, further, noise components are overlapped on the detection signal function, and the error range of the position of the extreme value which is specified based on the amplitude data only tends to increase.
Even if it is attempted to utilize the data related to the phase, the phase includes a phase shift quantity due to the reflection by the object to be measured, which makes it, usually, difficult to correctly find a phase shift quantity and hence it is necessary to be considered as being an unknown quantity. In the detection signal function, further, changes in phase constitute a periodic function of a short period as compared to changes in amplitude, and there exist a plurality of phases having a predetermined relationship to the phase shift quantity even near the extreme value of amplitude. Therefore, it is difficult to specify the extreme value of amplitude though the phase shift quantity can be correctly found.